The speed of a motor (e.g. of a fan) may be controlled by linear regulation. Thereby the DC voltage across the motor is adjusted.
An alternative way to control the speed of a motor is by direct pulse width modulation. In that case the motor's power supply is modulated by a PWM waveform. Therefore a pass transistor may be positioned between the power supply and the VDD pin of the motor (the high side of the motor), or between the ground pin of the motor and the ground (the low side of the motor). By controlling the gate of this transistor the current path for the motor can be switched on or off. Thus, a PWM waveform can be applied.
Using a p-channel MOSFET (or a PNP) as pass device on the positive power supply wire of the motor requires a level shifter on the PWM signal to swing up to the motor voltage to drive the pass device. In that case, the tachometer or locked-rotor feedback unit on the motordriver can be pulled up to the supply of a remote controlling fan monitoring IC.
Alternatively an n-channel MOSFET (or an NPN) on the ground wire can be used instead. This allows the pass device to be driven by a 3.3V or 5V logic-level PWM signal, which is more easy to implement. However this implies that the ground of the feedback unit will be floating during the PWM OFF period, which complicates the feedback implementation.
There are existing different prior art PWM motor speed controllers that are compatible with either n-channel or p-channel motor drive.
An example of a prior art 4-wire motor solution is shown in FIG. 1. In this figure, a digital PWM signal is used to control the motordriver (e.g. fandriver). The motordriver provides speed information back via the FG (frequency generator or tachometer signal) feedback unit.
An example of a prior art 3-wire motor solution is shown in FIG. 2. In this example, the PWM signal of the fan monitor controls a high side switch in the supply line, and controls the motor speed by adjusting the duty cycle of the supply ON time. The FG feedback unit provides speed information to the remote fan monitoring IC.
An example of a prior art 2-wire solution is shown in FIG. 3. In this example, the fan monitor regulates the motor speed based on the sensed temperature of the object, which is being cooled by the motor. In this case, the fan monitor has no only indirect feedback of the motor speed. The fan monitor does not control the motor noise, and can only indirectly, through the temperature sensing guess if the motor is still working or not.
In the 2 and 3-wire solution, the motordriver will shut down during the PWM OFF period as soon as the VDD drops below the power down level of the motordriver. When the supply is switched on again, the motordriver will be initialized, and switch on the output driver according to the hall sensor information, with 100% duty cycle (ON/OFF control).
The initialization of the motordriver may take some time, during which the motordriver is not operating in a proper way. Also, no intelligent soft-switching algorithms can be applied which requires speed information. Finally, at low speeds the low BEMF will cause significant peak currents to flow, which lead to reduced lifetime and to audible noise.
By use of linear controlled switches the supply can be regulated in an analog way, avoiding the above disadvantages. However at cost of efficiency. Also, this method is limited in speed range. The range between the minimum speed and the maximum speed is limited by the minimum supply voltage. For instance, the minimum operating voltage for 12V motordrivers is as low as 2.5V. Even if at some extra motordriver cost the supply range could be reduced to 1.8V, still the ratio to the 12V supply voltage is more than 10%, which is the range which can be achieved with a 4-wire motor solution.
In summary, problems might occur when driving the motor at lower PWM frequencies (e.g. at frequencies below 30 Hz). As frequencies decrease, an audible change in the motor's speed during the on and off periods of the PWM waveform may be noticed. Moreover, the PWM ON period (the period during which the motor is powered) may become too short to allow the motor's internal electronics to turn on and begin driving the motor. Hence, motor reliability problems may occur when decreasing the PWM ON period.
In view of these problems, there is still room for improvement in motordrivers for PWM driven motors.